Silicon is the basic building block material of most microelectronic devices. Other micro-scale devices such as microelectro-mechanical devices (MEMs), and micro-optic devices, are also generally made of silicon. These devices are used in virtually all modern electronic products. The raw silicon material used in making these types of microscopic devices is ordinarily provided in the form of thin flat polished wafers.
Porous silicon is a form of silicon having tiny openings or pores. These pores can absorb and emit light. This allows porous silicon devices to interact with light and electronic devices in many useful ways. Porous silicon also has a very large surface area and acts as a strong adsorbent. These properties make porous silicon useful in mass spectrometry, micro-fluidic devices, sensors, fuel cell electrodes, optical, chemical and mechanical filters, biochips and biosensors, fuses for airbags, and various other products.
The porous silicon material itself may also be used as a porous and/or solvable substrate, for example in diagnostic or therapeutic products. Accordingly, porous silicon is increasingly becoming an important material in a wide range of products and technologies.
Porous silicon is generally manufactured in an electro-chemical etching process. A silicon wafer is typically exposed to an electrolyte including concentrated hydrofluoric acid (HF). The electrolyte on one side of the wafer is sealed off from the electrolyte on the other side of the wafer. Electrical current is passed through the electrolyte on each side, making one side the cathode and the other side the anode. The silicon wafer may optionally be exposed to light during this process. The process etches pores in the wafer. The pores are microscopic. A 150 mm diameter wafer may have more than 1 billion pores after electro-chemical processing. Porous silicon may be formed by starting with p-type or n-type silicon, and then forming porous silicon in an electro-chemical process.
Although various types of porous silicon machines or processors have been used, disadvantages remain in performance, reliability, speed, and other design parameters. HF is highly corrosive and toxic. Accordingly, it must be carefully contained within the processor. Since HF will react with virtually all metals, metals cannot effectively be used in areas of the processor that may come into contact with HF. Moreover, even the smallest of amount of interaction between the HF in the electrolyte and metal can contaminate the wafer. The uniform processing required to consistently produce high quality porous silicon also requires uniform electrical current flow through the electrolyte. Achieving uniform current flow is affected by the design of the processor and may be challenging to achieve.
Some porous silicon processors require illumination of the wafer, creating still further design challenges. In these processors, highly uniform and very bright lighting is desired to achieve high quality porous silicon. Uniform current flow is also a significant factor. However, the size, shape and location of an anode electrode designed for uniform current flow may interfere with the lighting, especially in a compact processor design. Accordingly, achieving both uniform lighting and uniform current flow in a porous silicon wafer processor may be difficult.
Existing illuminated porous silicon processors generally use large, high power tungsten halogen lamps, or similar types of lamps. Typically, these lamps are positioned relatively far from the wafer. As a result, they tend to illuminate not only the wafer, but also a large area around the wafer. Consequently, they may consume excessive electrical power and generate excessive heat. Excessive heat may be disadvantageous, as it can affect the process liquid, which often includes a solvent such as isopropyl alcohol. Powering the lamps using the low voltages generally associate with solvent containing process liquids can create other design problems as well.
Existing processors have offered only varying results in the face of these engineering design challenges. In view of these factors, improved methods, processors and systems for making porous silicon are needed.